In the field of secondary ion mass spectrometry depth profiling, a technique for analyzing the chemical components of the surface of an object, a primary ion beam is directed onto the surface which is being analyzed. The primary ion beam is scanned in a raster pattern over the surface. As the beam is scanned, the surface is sputter etched and some of the atoms leave as secondary ions. The secondary ions are then introduced into a mass spectrometer and the ion can then be identified.
In the normal course of operating such an instrument, the beam scanning produces a small crater in the surface being investigated. As the crater becomes deeper, the beam hits the side wall of the crater. This causes some of the side wall material to be sputtered into the crater. Thereafter, the material is again sputtered and detected by the mass spectrometer thus creating the mistaken impression that the ion came from the crater depth then being sputtered as opposed to being from the side wall at some other depth.
As a result of the sputtering of the crater side wall into the crater, an error or background signal is produced in a secondary ion mass spectrometer depth profile. The background signal thus serves to limit the range over which accurate measurements can be made.
The prior art has been to prevent this by simply increasing the primary beam scan magnification, thereby reducing the raster scan size, when such crater wall sputtering became a significant source of background signal. The undesirable consequences of this procedure are that (1) the sputter rate changes (increases) due to the decrease in sputtered area, and (2) the probability that a sample atom will be sputtered as an ion (as opposed to a neutral) may change due to the change in the local exposure to a reactive gas (in the case of inert ion bombardment with reactive gas exposure for ion yield enhancement). Both of these effects make data quantitation more difficult.
In view of the above mentioned problems, it is the principal objective of the present invention to eliminate one source of background signal that manifests itself in a secondary ion mass spectrometer depth profile.